I'm making a mock up of a 12" square cake but I'm probably about to ask the most stupid question ever, so I apologize beforehand. How do you measure out a 12" square cake tin? Is 12" on a diagonal, the way you measure the size of a computer screen or is it 12" on each side?

12" on each side.... the inside part of the pan will be 12" if you measure the outside you'll probably get 12.25 or something......